Raul,upon reading the same Stereophile article from the 1994 archives, I found this rebuttal letter at the end of the article regarding frequency response variations of tube amplifiers with regard to speaker impedance changes and damping factors.
From the Stereophile archives
"A letter in response appeared in April 1994 (Vol.17 No.4):
Thomas J. Norton's "Questions of Impedance Interaction" in January (p.109) showed that a ghostly echo of a loudspeaker's impedance modulus can be imposed on its frequency response by virtue of an amplifier's source impedance acting as the top limb of a potential divider. Mr. Norton illustrated the effect with some specific graphics, but the data can be usefully generalized by means of a simple rule arising from the ohmic arithmetic.
Assuming a worst-case situation of very large impedance undulations, with, for instance, an LF peak reaching 10 times the value of that characterizing the lower-mid region, the rule runs as follows: To confine frequency-response changes within an amplitude band of 1dB, the amplifier's source impedance (plus the resistance of its connecting cable) must be less than an eighth of the speaker's impedance at the latter's lowest point, and less than a sixteenth for a band of 0.5dB.
These criteria also satisfy damping requirements for practical purposes, since the resistance of a speaker's voice-coil is effectively in series with the amplifier feed, so that once the latter falls below about one quarter of the coil resistance, there can be no worthwhile improvement. Perceived changes in bass "softness" between transistor and tube amplifiers are thus more likely to be due to LF impedance "response ghosts" than to the damping-factor, as such. Huge ratios for this parameter make impressive reading in technical specifications (like ultra-low distortion figures), but are so far along the relevant asymptotic curve as to be useless for audio purposes..John Crabbe, Todmorden, Lancashire, England "
End of Stereophile excerpt.
So, as you can see, there is no consensus that this lower damping factor in certain tube amps with low/no negative feedback in conjunction with certain speakers will actually have the frequency response effects that you refer to as "equalizers". As long as the damping factor remains above 4, then the "equalizer effects" that you refer to are insignificant according to Mr. Crabbe(and others). Many tube amplifiers have output impedances of less than 2 ohms, and many "tube friendly" speakers have impedance curves which remain at(or above) 8 ohms, so this can provide the necessary damping factors of >4 that are required for linear response within about 1 db.
Additionally, it has been known for many years that adding negative feedback to improve damping factor and reduce measured distortion(which is common in both tube amps and SS amps) can be deleterious to the musical sound quality. When applying negative feedback to a sinewave test tone, it shows reduced measured distortion and also improves damping factor. But when playing music which is non-constant, the feedback loop actually can increase music distortion because the feedback added is actually phase shifted(due to the length of the feedback loop), and adds feedback from a previously played part of the signal to an upcoming part of the signal, thus making a big mess. Now, to be fair, engineers have used short feedback loops and other techniques to minimize this problem, but it still can and does exist. This is why some amp makers are adamant about making "no feedback" amplifiers.
So yes, I understand your concerns about the tube amps becoming "equalizers" in theory, but in practice(with a correctly matched system) this concern is not founded in reality. In a poorly matched system, this could be a problem, but all kinds of problems can arise in a poorly matched system. Your "broad brush" application of this concept to all tube amp/speaker systems is misleading at the very least, and disingenuous at worst.
While SS amps with negative feedback and high damping factors may have certain good aspects, there are also downsides to that type of design, as I have just described. Many SS amp makers make strong efforts to minimize or even eliminate feedback in their amps, so that these problems do not noticeably degrade their amps' performance. I agree that while the naturally low output impedance of SS amps, and even the use of negative feedback, can provide a very impressive damping factor number, the overall benefits of this become "asymptotically small" after a small basic damping factor number(>4) is achieved. The main benefit comes from being able to choose from a wider range of speakers which may exhibit low nominal impedances and low impedance swings that may strain amps with higher output impedances. It can be a useful thing in some circumstances. But, matching the amp to the speaker can and does make the difference, and as long as the user observes proper matching, this "equalizer" phenomenon is not a real concern.
As we now see, there are plusses and minuses to various types of designs, and it is the user's responsibility to configure his system to the best effect.
Regards, and enjoy the music.
TWL
